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This chapter introduces and explains the primary functions and features of a router and explains the process for connecting and configuring devices to the router. It continues by describing the process by which routers manage packets, determine data paths, and build routing tables.

Objectives

Upon completion of this chapter, you will be able to answer the following questions:

• What are the primary functions and features of a router?
• How do you connect devices for a small routed network?
• Can you configure basic settings on a router to route between two directly connected networks?
• How can you verify connectivity between two networks that are directly connected to a router?
• How do routers encapsulate and de-encapsulate packets when switching packets between directly connected interfaces?
• How do routers determine the best path?
• How do routers build a routing table of directly connected networks?
• How do routers build a routing table using static routes?
• How do routers build a routing table using a dynamic routing protocol?

Key Terms

This chapter uses the following key terms. You can find the definitions in the Glossary.

• default gateway
• physical topology
• logical topology
• availability
• scalability
• reliability
• Random Access Memory (RAM)
• Read-Only Memory (ROM)
• Non-Volatile Random Access Memory (NVRAM)
• Flash
• process switching
• fast switching
• Cisco Express Forwarding (CEF)
• IP address
• subnet mask
• topology diagram
• addressing table
• statically assigned IP address
• dynamically assigned IP address
• console cable
• terminal emulation software
• switched virtual interface (SVI)
• High-Speed WAN Interface Card (HWIC)
• loopback interface
• directly connected network
• remote network
• Gateway of Last Resort
• metric
• best path
• equal cost load balancing
• unequal cost load balancing
• administrative distance
• routing table

Introduction (1.0.1.1)

Networks allow people to communicate, collaborate, and interact in many ways. Networks are used to access web pages, talk using IP telephones, participate in video conferences, compete in interactive gaming, shop using the Internet, complete online coursework, and more.

At the core of the network is the router. A router connects one network to another network. The router is responsible for the delivery of packets across different networks. The destination of the IP packet might be a web server in another country or an email server on the local-area network.

The router uses its routing table to determine the best path to use to forward a packet. It is the responsibility of the routers to deliver those packets in a timely manner. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.

When a host sends a packet to a device on a different IP network, the packet is forwarded to the default gateway because a host device cannot communicate directly with devices outside of the local network. The default gateway is the destination that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.

This chapter will also answer the question, “What does a router do with a packet received from one network and destined for another network?” Details of the routing table will be examined, including connected, static, and dynamic routes.

Because the router can route packets between networks, devices on different networks can communicate. This chapter will introduce the router, its role in the networks, its main hardware and software components, and the routing process.

Initial Configuration of a Router (1.1)

A router is essentially a special-purpose computer with an internetwork operating system optimized for the purpose of routing and securing networks. This section will examine the functions of a router and how a router determines the best path. It will also review the command-line interface (CLI) commands required to configure the base settings of a router.

Characteristics of a Network (1.1.1.1)

Networks have had a significant impact on our lives. They have changed the way we live, work, and play.

Networks allow us to communicate, collaborate, and interact in ways we never did before. We use the network in a variety of ways, including web applications, IP telephony, video conferencing, interactive gaming, electronic commerce, education, and more.

There are many terms, key structures, and performance-related characteristics that are referred to when discussing networks. These include:

• Topology: There are physical and logical topologies. The physical topology is the arrangement of the cables, network devices, and end systems. It describes how the network devices are actually interconnected with wires and cables. The logical topology is the path over which the data is transferred in a network. It describes how the network devices appear connected to network users.
• Speed: Speed is a measure of the data rate in bits per second (b/s) of a given link in the network.
• Cost: Cost indicates the general expense for purchasing of network components, and installation and maintenance of the network.
• Security: Security indicates how protected the network is, including the information that is transmitted over the network. The subject of security is important, and techniques and practices are constantly evolving. Consider security whenever actions are taken that affect the network.
• Availability: Availability is a measure of the probability that the network is available for use when it is required.
• Scalability: Scalability indicates how easily the network can accommodate more users and data transmission requirements. If a network design is optimized to only meet current requirements, it can be very difficult and expensive to meet new needs when the network grows.
• Reliability: Reliability indicates the dependability of the components that make up the network, such as the routers, switches, PCs, and servers. Reliability is often measured as a probability of failure or as the mean time between failures (MTBF).

These characteristics and attributes provide a means to compare different networking solutions.

Why Routing? (1.1.1.2)

How does clicking a link in a web browser return the desired information in mere seconds? Although there are many devices and technologies collaboratively working together to enable this, the primary device is the router. Stated simply, a router connects one network to another network.

Communication between networks would not be possible without a router determining the best path to the destination and forwarding traffic to the next router along that path. The router is responsible for the routing of traffic between networks.

When a packet arrives on a router interface, the router uses its routing table to determine how to reach the destination network. The destination of the IP packet might be a web server in another country or an email server on the local-area network. It is the responsibility of routers to deliver those packets efficiently. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.

Routers Are Computers (1.1.1.3)

Most network capable devices (i.e., computers, tablets, and smartphones) require the following components to operate:

• Central processing unit (CPU)
• Operating system (OS)
• Memory and storage (RAM, ROM, NVRAM, Flash, hard drive)

A router is essentially a specialized computer. It requires a CPU and memory to temporarily and permanently store data to execute operating system instructions, such as system initialization, routing functions, and switching functions.

Routers store data using:

• Random Access Memory (RAM): Provides temporary storage for various applications and processes, including the running IOS, the running configuration file, various tables (i.e., IP routing table, Ethernet ARP table), and buffers for packet processing. RAM is referred to as volatile because it loses its contents when power is turned off.
• Read-Only Memory (ROM): Provides permanent storage for bootup instructions, basic diagnostic software, and a limited IOS in case the router cannot load the full featured IOS. ROM is firmware and referred to as non-volatile because it does not lose its contents when power is turned off.
• Non-Volatile Random Access Memory (NVRAM): Provides permanent storage for the startup configuration file (startup-config). NVRAM is non-volatile and does not lose its contents when power is turned off.
• Flash: Provides permanent storage for the IOS and other system-related files. The IOS is copied from flash into RAM during the bootup process. Flash is non-volatile and does not lose its contents when power is turned off.

Table 1-1 provides a summary of the types of router memory, their volatility, and examples of what is stored in each.

Table 1-1 Router Memory

Memory	Volatile/Non-Volatile	Stores
RAM	Volatile	Running IOS Running configuration file IP routing and ARP tables Packet buffer
ROM	Non-volatile	Bootup instructions Basic diagnostic software Limited IOS
NVRAM	Non-volatile	Startup configuration file
Flash	Non-volatile	IOS file Other system files

Unlike a computer, a router does not have video adapters or sound card adapters. Instead, routers have specialized ports and network interface cards to interconnect devices to other networks. Figure 1-1 displays the back panel of a Cisco 1941 ISRG2 and identifies those special ports and interfaces.

Routers Interconnect Networks (1.1.1.4)

Most users are unaware of the presence of numerous routers on their own network or on the Internet. Users expect to be able to access web pages, send emails, and download music, regardless of whether the server accessed is on their own network or on another network. Networking professionals know that it is the router that is responsible for forwarding packets from network to network, from the original source to the final destination.

A router connects multiple networks, which means that it has multiple interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. The interface that the router uses to forward the packet may be the final destination, or it may be a network connected to another router that is used to reach the destination network.

Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both local-area networks (LANs) and wide-area networks (WANs). LANs are commonly Ethernet networks that contain devices, such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet service provider (ISP) network.

Notice that each site in Figure 1-2 requires the use of a router to interconnect to other sites. Even the Home Office requires a router. In this topology, the router located at the Home Office is a specialized device that performs multiple services for the home network.

Routers Choose Best Paths (1.1.1.5)

The primary functions of a router are to:

• Determine the best path to send packets
• Forward packets toward their destination

The router uses its routing table to determine the best path to use to forward a packet. When the router receives a packet, it examines the destination address of the packet and uses the routing table to search for the best path to that network. The routing table also includes the interface to be used to forward packets for each known network. When a match is found, the router encapsulates the packet into the data link frame of the outgoing or exit interface, and the packet is forwarded toward its destination.

It is possible for a router to receive a packet that is encapsulated in one type of data link frame, and to forward the packet out of an interface that uses a different type of data link frame. For example, a router may receive a packet on an Ethernet interface, but must forward the packet out of an interface configured with the Point-to-Point Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium to which it connects. The different data link technologies that a router can connect to include Ethernet, PPP, Frame Relay, DSL, cable, and wireless (802.11, Bluetooth).

Packet Forwarding Mechanisms (1.1.1.6)

Routers support three packet-forwarding mechanisms:

• Process switching: An older packet-forwarding mechanism still available for Cisco routers. When a packet arrives on an interface, it is forwarded to the control plane, where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets. This process-switching mechanism is very slow and rarely implemented in modern networks. Figure 1-3 illustrates how packets are process-switched.

  

  Figure 1-3 Process Switching

• Fast switching: This is a common packet-forwarding mechanism which uses a fast-switching cache to store next-hop information. When a packet arrives on an interface, it is forwarded to the control plane, where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is also stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention. Figure 1-4 illustrates how packets are fast-switched.

  

  Figure 1-4 Fast Switching

• Cisco Express Forwarding (CEF): CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB) and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups and next-hop information for routes, including the interface and Layer 2 information. Cisco Express Forwarding is the fastest forwarding mechanism and the preferred choice on Cisco routers. Figure 1-5 illustrates how packets are forwarded using CEF.

  

  Figure 1-5 Cisco Express Forwarding

Figures 1-3 to 1-5 illustrate the differences between the three packet-forwarding mechanisms. Assume a traffic flow consisting of five packets all going to the same destination. As shown in Figure 1-3, with process switching, each packet must be processed by the CPU individually. Contrast this with fast switching, as shown in Figure 1-4. With fast switching, notice how only the first packet of a flow is process-switched and added to the fast-switching cache. The next four packets are quickly processed based on the information in the fast-switching cache. Finally, in Figure 1-5, CEF builds the FIB and adjacency tables, after the network has converged. All five packets are quickly processed in the data plane.

A common analogy used to describe the three packet-forwarding mechanisms is as follows:

• Process switching solves a problem by doing math long hand, even if it is the identical problem.
• Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems.
• CEF solves every possible problem ahead of time in a spreadsheet.

Connect Devices (1.1.2)

In this section, you will see how accessing a network involves connecting hosts and infrastructure devices with IP addresses, subnet masks, and default gateways. This section will also introduce how to configure the initial settings of a switch.

Connect to a Network (1.1.2.1)

Network devices and end users typically connect to a network using a wired Ethernet or wireless connection. Refer to the sample reference topology in Figure 1-6. The LANs in the figure serve as an example of how users and network devices could connect to networks.

Home Office devices can connect as follows:

• Laptops and tablets connect wirelessly to a home router.
• A network printer connects using an Ethernet cable to the switch port on the home router.
• The home router connects to the service provider cable modem using an Ethernet cable.
• The cable modem connects to the Internet service provider (ISP) network.

The Branch site devices connect as follows:

• Corporate resources (i.e., file servers and printers) connect to Layer 2 switches using Ethernet cables.
• Desktop PCs and voice over IP (VoIP) phones connect to Layer 2 switches using Ethernet cables.
• Laptops and smartphones connect wirelessly to wireless access points (WAPs).
• The WAPs connect to switches using Ethernet cables.
• Layer 2 switches connect to an Ethernet interface on the edge router using Ethernet cables. An edge router is a device that sits at the edge or boundary of a network and routes between that network and another, such as between a LAN and a WAN.
• The edge router connects to a WAN service provider (SP).
• The edge router also connects to an ISP for backup purposes.

The Central site devices connect as follows:

• Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables.
• Layer 2 switches connect redundantly to multilayer Layer 3 switches using Ethernet fiber-optic cables (orange connections).
• Layer 3 multilayer switches connect to an Ethernet interface on the edge router using Ethernet cables.
• The corporate website server is connected using an Ethernet cable to the edge router interface.
• The edge router connects to a WAN SP.
• The edge router also connects to an ISP for backup purposes.

In the Branch and Central LANs, hosts are connected either directly or indirectly (via WAPs) to the network infrastructure using a Layer 2 switch.

Default Gateways (1.1.2.2)

To enable network access, devices must be configured with IP address information to identify the appropriate:

• IP address: Identifies a unique host on a local network
• Subnet mask: Identifies with which network subnet the host can communicate
• Default gateway: Identifies the router to send a packet to when the destination is not on the same local network subnet

When a host sends a packet to a device that is on the same IP network, the packet is simply forwarded out of the host interface to the destination device.

When a host sends a packet to a device on a different IP network, then the packet is forwarded to the default gateway, because a host device cannot communicate directly with devices outside of the local network. The default gateway is the destination that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.

The default gateway is usually the address of the interface on the router connected to the local network. The router maintains routing table entries of all connected networks as well as entries of remote networks, and determines the best path to reach those destinations.

For example, if PC1 sends a packet to the Web Server located at 172.16.1.99, it would discover that the Web Server is not on the local network and it, therefore, must send the packet to the Media Access Control (MAC) address of its default gateway. The packet protocol data unit (PDU) in Figure 1-7 identifies the source and destination IP and MAC addresses.

Document Network Addressing (1.1.2.3)

When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should identify:

• Device names
• Interfaces used in the design
• IP addresses and subnet masks
• Default gateway addresses

This information is captured by creating two useful network documents:

• Topology diagram: Provides a visual reference that indicates the physical connectivity and logical Layer 3 addressing. Often created using software, such as Microsoft Visio.
• Addressing table: A table that captures device names, interfaces, IPv4 addresses, subnet masks, and default gateway addresses.

Figure 1-8 displays the sample topology diagram, while Table 1-2 provides a sample addressing table for the topology.

Table 1-2 Addressing Table

Device	Interface	IP Address	Subnet Mask	Default Gateway
R1	Fa0/0	192.168.1.1	255.255.255.0	N/A
	S0/0/0	192.168.2.1	255.255.255.0	N/A
R2	Fa0/0	192.168.3.1	255.255.255.0	N/A
	S0/0/0	192.168.2.1	255.255.255.0	N/A
PC1	N/A	192.168.1.10	255.255.255.0	192.168.1.1
PC2	N/A	192.168.1.10	255.255.255.0	192.168.3.1

Enable IP on a Host (1.1.2.4)

A host can be assigned its IP address information in one of two ways. A host can get a:

• Statically Assigned IP Address: The host is manually assigned the correct IP address, subnet mask, and default gateway. The DNS server IP address can also be configured.
• Dynamically Assigned IP Address: IP address information is provided by a server using the Dynamic Host Configuration Protocol (DHCP). The DHCP server provides a valid IP address, subnet mask, and default gateway for end devices. Other information may be provided by the server.

Figures 1-9 and 1-10 provide static and dynamic IPv4 address configuration examples.

Statically assigned addresses are commonly used to identify specific network resources, such as network servers and printers. They can also be used in smaller networks with few hosts. However, most host devices acquire their IPv4 address information by accessing a DHCP server. In large enterprises, dedicated DHCP servers providing services to many LANs are implemented. In a smaller branch or small office setting, DHCP services can be provided by a Cisco Catalyst switch or a Cisco ISR.

Device LEDs (1.1.2.5)

Host computers connect to a wired network using a network interface and RJ-45 Ethernet cable. Most network interfaces have one or two LED link indicators next to the interface. Typically, a green LED means a good connection while a blinking green LED indicates network activity.

If the link light is not on, then there may be a problem with either the network cable or the network itself. The switch port where the connection terminates would also have an LED indicator lit. If one or both ends are not lit, try a different network cable.

Similarly, network infrastructure devices commonly use multiple LED indicators to provide a quick status view. For example, a Cisco Catalyst 2960 switch has several status LEDs to help monitor system activity and performance. These LEDs are generally lit green when the switch is functioning normally and lit amber when there is a malfunction.

Cisco ISRs use various LED indicators to provide status information. The LEDs on the router help the network administrator conduct some basic troubleshooting. Each device has a unique set of LEDs. Consult the device-specific documentation for an accurate description of the LEDs.

The LEDs of the Cisco 1941 router shown in Figure 1-11 are explained in Table 1-3.

Table 1-3 Description of the Cisco 1941 LEDs

#	Port	LED	Color	Description
1	GE0/0 and GE0/1	S (Speed)	1 blink + pause	Port operating at 10 Mb/s
			2 blink + pause	Port operating at 100 Mb/s
			3 blink + pause	Port operating at 1000 Mb/s
		L (Link)	Green	Link is active
			Off	Link is inactive
2	Console	EN	Green	Port is active
			Off	Port is inactive
3	USB	EN	Green	Port is active
			Off	Port is inactive

Console Access (1.1.2.6)

In a production environment, infrastructure devices are commonly accessed remotely using Secure Shell (SSH) or HyperText Transfer Protocol Secure (HTTPS). Console access is really only required when initially configuring a device, or if remote access fails.

Console access requires:

• Console cable: RJ-45-to-DB-9 console cable
• Terminal emulation software: Tera Term, PuTTY, HyperTerminal

The cable is connected between the serial port of the host and the console port on the device. Most computers and notebooks no longer include built-in serial ports. If the host does not have a serial port, the USB port can be used to establish a console connection. A special USB-to-RS-232 compatible serial port adapter is required when using the USB port.

The Cisco ISR G2 supports a USB serial console connection. To establish connectivity, a USB Type-A to USB Type-B (mini-B USB) is required, as well as an operating system device driver. This device driver is available from . Although these routers have two console ports, only one console port can be active at a time. When a cable is plugged into the USB console port, the RJ-45 port becomes inactive. When the USB cable is removed from the USB port, the RJ-45 port becomes active.

Table 1-4 summarizes the console connection requirements, while Figure 1-12 displays the various ports and cables required.

Table 1-4 Console Connection Requirements

Port on Computer	Cable Required	Port on ISR	Terminal Emulation
Serial Port	RJ-45 to DB-9 Console Cable	RJ-45 Console Port	Tera Term PuTTY
USB Type-A Port	USB to RS-232 compatible serial port adapter Adapter may require a software driver RJ-45 to DB-9 Console Cable
	USB Type-A to USB Type-B (Mini-B USB) A device driver is required and available from Cisco.com	USB Type-B(Mini-B USB)

Enable IP on a Switch (1.1.2.7)

Network infrastructure devices require IP addresses to enable remote management. Using the device IP address, the network administrator can remotely connect to the device using Telnet, SSH, HTTP, or HTTPS.

A switch does not have a dedicated interface to which an IP address can be assigned. Instead, the IP address information is configured on a virtual interface called a switched virtual interface (SVI).

The steps to configure the basic settings on a switch are as follows:

Step 1. Name the device.

Step 2. Configure the SVI. This makes the switch accessible for network management.

Step 3. Enable the SVI.

Step 4. Configure the default gateway for the switch. Packets generated by the switch and destined for an address other than its management network segment will be forwarded to this address. This default gateway is used by the switch only for the packets it generates, not any hosts connected to the switch.

For example, the following commands would configure the management VLAN interface and default gateway of switch S1 shown in Figure 1-13.

S1(config)# interface vlan 1
S1(config-if)# ip address 192.168.10.2 255.255.255.0
S1(config-if)# no shutdown
%LINK-5-CHANGED: Interface Vlan1, changed state to up
S1(config-if)# exit
S1(config)#
S1(config)# ip default-gateway 192.168.10.1
S1(config)#

In the example, the switch SVI is configured and enabled with the IP address 192.168.10.2/24 and a default gateway of the router located at 192.168.10.1. Packets generated by the switch and destined for an address outside of the 192.168.1.0/24 network segment will be forwarded to this address. In the example, the address is that of the G0/0 interface of R1.

Basic Settings on a Router (1.1.3)

The basic addressing and configuration of Cisco devices was covered in either the Introduction to Networks or Network Basics course. However, we will spend some time reviewing these topics as well as preparing you for the hands-on lab experience in this course.

Configure Basic Router Settings (1.1.3.1)

Cisco routers and Cisco switches have many similarities. They support a similar modal operating system, similar command structures, and many of the same commands. In addition, both devices have similar initial configuration steps.

When initially configuring a Cisco switch or router, the following steps should be executed:

• Step 1. Name the device. This changes the router prompt and helps distinguish the device from others.
• Step 2. Secure management access. Specifically, secure the privileged EXEC, user EXEC, and Telnet access, and encrypt passwords to their highest level.
• Step 3. Configure a banner. Although optional, this is a recommended step to provide legal notice to anyone attempting to access the device.
• Step 4. Save the configuration.

For example, the following commands would configure the basic settings for router R1 shown in Figure 1-14.

Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# hostname R1
R1(config)#
R1(config)# enable secret class
R1(config)#
R1(config)# line console 0
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# exit
R1(config)#
R1(config)# line vty 0 4
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# exit
R1(config)#
R1(config)# service password-encryption
R1(config)#
R1(config)# banner motd $ Authorized Access Only! $
R1(config)# end
R1#
R1# copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
R1#

Configure an IPv4 Router Interface (1.1.3.2)

One distinguishing feature between switches and routers is the type of interfaces supported by each. For example, Layer 2 switches support LANs and, therefore, have multiple FastEthernet or Gigabit Ethernet ports.

Routers support LANs and WANs and can interconnect different types of networks; therefore, they support many types of interfaces. For example, G2 ISRs have one or two integrated Gigabit Ethernet interfaces and High-Speed WAN Interface Card (HWIC) slots to accommodate other types of network interfaces, including serial, DSL, and cable interfaces.

To be available, an interface must be:

• If using IPv4, configured with an address and a subnet mask: Use the ip address ip-address subnet-mask interface configuration command.
• Activated: By default, LAN and WAN interfaces are not activated (shutdown). To enable an interface, it must be activated using the no shutdown command. (This is similar to powering on the interface.) The interface must also be connected to another device (a hub, a switch, or another router) for the physical layer to be active.

Optionally, the interface could also be configured with a short description. It is good practice to configure a description on each interface. The description text is limited to 240 characters. On production networks, a description can be helpful in troubleshooting by providing information about the type of network to which the interface is connected. If the interface connects to an ISP or service carrier, it is helpful to enter the third-party connection and contact information.

Depending on the type of interface, additional parameters may be required. For example, in the lab environment, the serial interface connecting to the serial cable end labeled DCE must be configured with the clock rate command.

The steps to configure an IPv4 interface on a router are:

• Step 1. Add a description. Although optional, it is a necessary component for documenting a network.
• Step 2. Configure the IPv4 address.
• Step 3. Configure a clock rate on Serial interfaces. This is only necessary on the DCE device in our lab environment and does not apply to Ethernet interfaces.
• Step 4. Enable the interface.

For example, the following commands would configure the three directly connected interfaces of router R1 shown in Figure 1-14 (in the previous section):

R1(config)# interface gigabitethernet 0/0
R1(config-if)# description Link to LAN 1
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)#
R1(config)# interface gigabitethernet 0/1
R1(config-if)# description Link to LAN 2
R1(config-if)# ip address 192.168.11.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)#
R1(config)# interface serial 0/0/0
R1(config-if)# description Link to R2
R1(config-if)# ip address 209.165.200.225 255.255.255.252
R1(config-if)# clock rate 128000
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)#

Configure an IPv6 Router Interface (1.1.3.3)

Configuring an IPv6 interface is similar to configuring an interface for IPv4. Most IPv6 configuration and verification commands in the Cisco IOS are very similar to their IPv4 counterparts. In many cases, the only difference uses ipv6 in place of ip in commands.

An IPv6 interface must be:

• Configured with IPv6 address and subnet mask: Use the ipv6 address ipv6-address/prefix-length [link-local | eui-64] interface configuration command.
• Activated: The interface must be activated using the no shutdown command.

Unlike IPv4, IPv6 interfaces will typically have more than one IPv6 address. At a minimum, an IPv6 device must have an IPv6 link-local address but will most likely also have an IPv6 global unicast address. IPv6 also supports the ability for an interface to have multiple IPv6 global unicast addresses from the same subnet. The following commands can be used to statically create a global unicast or link-local IPv6 address:

• ipv6 address ipv6-address/prefix-length: Creates a global unicast IPv6 address as specified.
• ipv6 address ipv6-address/prefix-length eui-64: Configures a global unicast IPv6 address with an interface identifier (ID) in the low-order 64 bits of the IPv6 address using the EUI-64 process.
• ipv6 address ipv6-address/prefix-length link-local: Configures a static link-local address on the interface that is used instead of the link-local address that is automatically configured when the global unicast IPv6 address is assigned to the interface or enabled using the ipv6 enable interface command. Recall, the ipv6 enable interface command is used to automatically create an IPv6 link-local address whether or not an IPv6 global unicast address has been assigned.

The steps to configure an IPv6 interface on a router are:

• Step 1. Add a description. Although optional, it is a necessary component for documenting a network.
• Step 2. Configure the IPv6 global unicast address. Configuring a global unicast address automatically creates a link-local IPv6 address.
• Step 3. Configure a link-local unicast address which automatically assigns a link-local IPv6 address and overrides any previously assigned address.
• Step 4. Configure a clock rate on Serial interfaces. This is only necessary on the DCE device in our lab environment and does not apply to Ethernet interfaces.
• Step 5. Enable the interface.

In the example topology shown in Figure 1-15, R1 must be configured to support the following IPv6 global network addresses:

• 2001:0DB8:ACAD:0001:/64 (2001:DB8:ACAD:1::/64)
• 2001:0DB8:ACAD:0002:/64 (2001:DB8:ACAD:2::/64)
• 2001:0DB8:ACAD:0003:/64 (2001:DB8:ACAD:3::/64)

When the router is configured using the ipv6 unicast-routing global configuration command, the router begins sending ICMPv6 Router Advertisement messages out the interface. This enables a PC connected to the interface to automatically configure an IPv6 address and to set a default gateway without needing the services of a DHCPv6 server. Alternatively, a PC connected to the IPv6 network can get its IPv6 address statically assigned, as shown in Figure 1-16. Notice that the default gateway address configured for PC1 is the IPv6 global unicast address of the R1 Gigabit Ethernet 0/0 interface.

For example, the following commands would configure the IPv6 global unicast addresses of the three directly connected interfaces of the R1 router shown in Figure 1-15:

R1# configure terminal
R1(config)# interface gigabitethernet 0/0
R1(config-if)# description Link to LAN 1
R1(config-if)# ipv6 address 2001:db8:acad:1::1/64
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)#
R1(config)# interface gigabitethernet 0/1
R1(config-if)# description Link to LAN 2
R1(config-if)# ipv6 address 2001:db8:acad:2::1/64
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)#
R1(config)# interface serial 0/0/0
R1(config-if)# description Link to R2
R1(config-if)# ipv6 address 2001:db8:acad:3::1/64
R1(config-if)# clock rate 128000
R1(config-if)# no shutdown
R1(config-if)#

Configure an IPv4 Loopback Interface (1.1.3.4)

Another common configuration of Cisco IOS routers is enabling a loopback interface.

The loopback interface is a logical interface internal to the router. It is not assigned to a physical port and can therefore never be connected to any other device. It is considered a software interface that is automatically placed in an “up/up” state, as long as the router is functioning.

The loopback interface is useful in testing and managing a Cisco IOS device because it ensures that at least one interface will always be available. For example, it can be used for testing purposes, such as testing internal routing processes, by emulating networks behind the router.

Additionally, the IPv4 address assigned to the loopback interface can be significant to processes on the router that use an interface IPv4 address for identification purposes, such as the Open Shortest Path First (OSPF) routing process. By enabling a loopback interface, the router will use the always available loopback interface address for identification, rather than an IP address assigned to a physical port that may go down.

The steps to configure a loopback interface on a router are:

• Step 1. Create the loopback interface using the interface loopback number global configuration command.
• Step 2. Add a description. Although optional, it is a necessary component for documenting a network.
• Step 3. Configure the IP address.

For example, the following commands configure a loopback interface of the R1 router shown in Figure 1-14 (shown earlier in the chapter):

R1# configure terminal
R1(config)# interface loopback 0
R1(config-if)# ip address 10.0.0.1 255.255.255.0
R1(config-if)# exit
R1(config)#

A loopback interface is always enabled and therefore does not require a no shutdown command. Multiple loopback interfaces can be enabled on a router. The IPv4 address for each loopback interface must be unique and unused by any other interface.

Verify Connectivity of Directly Connected Networks (1.1.4)

The first task to undertake once the basic settings and interfaces are configured is to verify and validate the configured settings. This is an important step and should be done before any other configurations are added to the router.

Verify Interface Settings (1.1.4.1)

There are several show commands that can be used to verify the operation and configuration of an interface. The following three commands are especially useful to quickly identify an interface status:

• show ip interface brief: Displays a summary for all interfaces, including the IPv4 address of the interface and current operational status.
• show ip route: Displays the contents of the IPv4 routing table stored in RAM. In Cisco IOS 15, active interfaces should appear in the routing table with two related entries identified by the code 'C' (Connected) or 'L' (Local). In previous IOS versions, only a single entry with the code 'C' will appear.
• show running-config interface interface-id: Displays the commands configured on the specified interface.

Figure 1-17 displays the output of the show ip interface brief command.

The output reveals that the LAN interfaces and the WAN link are all activated and operational as indicated by the Status of “up” and Protocol of “up.” A different output would indicate a problem with either the configuration or the cabling.

Figure 1-18 displays the output of the show ip route command.

Notice the three directly connected network entries and the three local host route interface entries. A local host route has an administrative distance of 0. It also has a /32 mask for IPv4, and a /128 mask for IPv6. The local host route is for routes on the router owning the IP address. It is used to allow the router to process packets destined to that IP.

Figure 1-19 displays the output of the show running-config interface command. The output displays the current commands configured on the specified interface.

The following two commands are used to gather more detailed interface information:

• show interfaces: Displays interface information and packet flow count for all interfaces on the device
• show ip interface: Displays the IPv4-related information for all interfaces on a router

Verify IPv6 Interface Settings (1.1.4.2)

The commands to verify the IPv6 interface configuration are similar to the commands used for IPv4.

The show ipv6 interface brief command in Figure 1-20 displays a summary for each of the interfaces.

The “up/up” output on the same line as the interface name indicates the Layer 1/Layer 2 interface state. This is the same as the Status and Protocol columns in the equivalent IPv4 command.

The output displays two configured IPv6 addresses per interface. One address is the IPv6 global unicast address that was manually entered. The other address, which begins with FE80, is the link-local unicast address for the interface. A link-local address is automatically added to an interface whenever a global unicast address is assigned. An IPv6 network interface is required to have a link-local address, but not necessarily a global unicast address.

The show ipv6 interface gigabitethernet 0/0 command output shown in Figure 1-21 displays the interface status and all of the IPv6 addresses belonging to the interface. Along with the link-local address and global unicast address, the output includes the multicast addresses assigned to the interface, beginning with prefix FF02.

The show ipv6 route command shown in Figure 1-22 can be used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table. The show ipv6 route command will only display IPv6 networks, not IPv4 networks.

Within the routing table, a ‘C’ next to a route indicates that this is a directly connected network. When the router interface is configured with a global unicast address and is in the “up/up” state, the IPv6 prefix and prefix length is added to the IPv6 routing table as a connected route.

The IPv6 global unicast address configured on the interface is also installed in the routing table as a local route, as indicated with an ‘L’ next to the route entry. The local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with the interface address of the router as the destination.

The ping command for IPv6 is identical to the command used with IPv4 except that an IPv6 address is used. As shown in Figure 1-23, the ping command is used to verify Layer 3 connectivity between R1 and PC1.

Other useful IPv6 verification commands include:

• show interface
• show ipv6 routers

Filter Show Command Output (1.1.4.3)

Commands that generate multiple screens of output are, by default, paused after 24 lines. At the end of the paused output, the --More-- text displays. Pressing Enter displays the next line and pressing the spacebar displays the next set of lines. Use the terminal length number command to specify the number of lines to be displayed. A value of 0 (zero) prevents the router from pausing between screens of output.

Another very useful feature that improves the user experience in the command-line interface (CLI) is the filtering of show output. Filtering commands can be used to display specific sections of output. To enable the filtering command, enter a pipe (|) character after the show command and then enter a filtering parameter and a filtering expression.

The filtering parameters that can be configured after the pipe include:

• section: Shows entire section that starts with the filtering expression
• include: Includes all output lines that match the filtering expression
• exclude: Excludes all output lines that match the filtering expression
• begin: Shows all the output lines from a certain point, starting with the line that matches the filtering expression

Figures 1-24 through 1-27 provide examples of the various output filters. The example in Figure 1-24 uses the pipe character and the section keyword.

The example in Figure 1-25 uses the pipe character and the include keyword.

The example in Figure 1-26 uses the pipe character and the exclude keyword.

The example in Figure 1-27 uses the pipe character and the begin keyword.

Command History Feature (1.1.4.4)

The command history feature is useful, because it temporarily stores the list of executed commands to be recalled.

To recall commands in the history buffer, press Ctrl+P or the Up Arrow key. The command output begins with the most recent command. Repeat the key sequence to recall successively older commands. To return to more recent commands in the history buffer, press Ctrl+N or the Down Arrow key. Repeat the key sequence to recall successively more recent commands.

By default, command history is enabled and the system captures the last 10 command lines in its history buffer. Use the show history privileged EXEC command to display the contents of the buffer.

It is also practical to increase the number of command lines that the history buffer records during the current terminal session only. Use the terminal history size user EXEC command to increase or decrease the size of the buffer.

For example, the following displays a sample of the terminal history size and show history commands:

R1# terminal history size 200
R1#
R1# show history
  show ip interface brief
  show interface g0/0
  show ip interface g0/1
  show ip route
  show ip route 209.165.200.224
  show running-config interface s0/0/0
  terminal history size 200
  show history
R1#

Routing Decisions (1.2)

The key to understanding the role of a router in the network is to understand that a router is a Layer 3 device responsible for forwarding packets. However, a router also operates at Layers 1 and 2.

Router Switching Function (1.2.1.1)

A primary function of a router is to forward packets toward their destination. This is accomplished by using a switching function, which is the process used by a router to accept a packet on one interface and forward it out of another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.

After the router has determined the exit interface using the path determination function, the router must encapsulate the packet into the data link frame of the outgoing interface.

What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:

• Step 1. De-encapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer.
• Step 2. Examines the destination IP address of the IP packet to find the best path in the routing table.
• Step 3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

As shown in Figure 1-28, devices have Layer 3 IPv4 addresses and Ethernet interfaces have Layer 2 data link addresses. For example, PC1 is configured with IPv4 address 192.168.1.10 and an example MAC address of 0A-10. As a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. However, the Layer 2 data link addresses change at every hop as the packet is de-encapsulated and re-encapsulated in a new frame by each router. It is very likely that the packet is encapsulated in a different type of Layer 2 frame than the one in which it was received. For example, an Ethernet encapsulated frame might be received by the router on a FastEthernet interface, and then processed to be forwarded out of a serial interface as a Point-to-Point Protocol (PPP) encapsulated frame.

Send a Packet (1.2.1.2)

In the animation in the online course, PC1 is sending a packet to PC2.

PC1 must determine if the destination IPv4 address is on the same network. PC1 determines its own subnet by doing an AND operation on its own IPv4 address and subnet mask. This produces the network address that PC1 belongs to. Next, PC1 does this same AND operation using the packet destination IPv4 address and the PC1 subnet mask.

If the destination network address is the same network as PC1, then PC1 does not use the default gateway. Instead, PC1 refers to its ARP cache for the MAC address of the device with that destination IPv4 address. If the MAC address is not in the cache, then PC1 generates an ARP request to acquire the address to complete the packet and send it to the destination. If the destination network address is on a different network, then PC1 forwards the packet to its default gateway.

To determine the MAC address of the default gateway, PC1 checks its ARP table for the IPv4 address of the default gateway and its associated MAC address.

If an ARP entry does not exist in the ARP table for the default gateway, PC1 sends an ARP request. Router R1 sends back an ARP reply. PC1 can then forward the packet to the MAC address of the default gateway, the Fa0/0 interface of router R1.

A similar process is used for IPv6 packets. Instead of the ARP process, IPv6 address resolution uses ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages. IPv6-to-MAC address mappings are kept in a table similar to the ARP cache, called the neighbor cache.

Forward to the Next Hop (1.2.1.3)

The following processes take place when R1 receives the Ethernet frame from PC1:

1. R1 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R1, therefore, copies the frame into its buffer.
2. R1 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
3. R1 de-encapsulates the Ethernet frame.
4. Because the destination IPv4 address of the packet does not match any of the directly connected networks of R1, R1 consults its routing table to route this packet. R1 searches the routing table for a network address that would include the destination IPv4 address of the packet as a host address within that network. In this example, the routing table has a route for the 192.168.4.0/24 network. The destination IPv4 address of the packet is 192.168.4.10, which is a host IPv4 address on that network.

The route that R1 finds to the 192.168.4.0/24 network has a next-hop IPv4 address of 192.168.2.2 and an exit interface of FastEthernet 0/1. This means that the IPv4 packet is encapsulated in a new Ethernet frame with the destination MAC address of the IPv4 address of the next-hop router.

Because the exit interface is on an Ethernet network, R1 must resolve the next-hop IPv4 address with a destination MAC address using ARP:

1. R1 looks up the next-hop IPv4 address of 192.168.2.2 in its ARP cache. If the entry is not in the ARP cache, R1 would send an ARP request out of its FastEthernet 0/1 interface and R2 would send back an ARP reply. R1 would then update its ARP cache with an entry for 192.168.2.2 and the associated MAC address.
2. The IPv4 packet is now encapsulated into a new Ethernet frame and forwarded out the FastEthernet 0/1 interface of R1.

The animation in the online course illustrates how R1 forwards the packet to R2.

Packet Routing (1.2.1.4)

The following processes take place when R2 receives the frame on its Fa0/0 interface:

1. R2 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R2, therefore, copies the frame into its buffer.
2. R2 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
3. R2 de-encapsulates the Ethernet frame.
4. Because the destination IPv4 address of the packet does not match any of the interface addresses of R2, R2 consults its routing table to route this packet. R2 searches the routing table for the destination IPv4 address of the packet using the same process R1 used.
5. The routing table of R2 has a route to the 192.168.4.0/24 network, with a next-hop IPv4 address of 192.168.3.2 and an exit interface of Serial 0/0/0. Because the exit interface is not an Ethernet network, R2 does not have to resolve the next-hop IPv4 address with a destination MAC address.
6. The IPv4 packet is now encapsulated into a new data link frame and sent out the Serial 0/0/0 exit interface.

When the interface is a point-to-point (P2P) serial connection, the router encapsulates the IPv4 packet into the proper data link frame format used by the exit interface (HDLC, PPP, etc.). Because there are no MAC addresses on serial interfaces, R2 sets the data link destination address to an equivalent of a broadcast (MAC address: FF:FF:FF:FF:FF:FF).

The animation in the online course illustrates how R2 forwards the packet to R3.

Reach the Destination (1.2.1.5)

The following processes take place when the frame arrives at R3:

1. R3 copies the data link PPP frame into its buffer.
2. R3 de-encapsulates the data link PPP frame.
3. R3 searches the routing table for the destination IPv4 address of the packet. The routing table has a route to a directly connected network on R3. This means that the packet can be sent directly to the destination device and does not need to be sent to another router.

Because the exit interface is a directly connected Ethernet network, R3 must resolve the destination IPv4 address of the packet with a destination MAC address:

1. R3 searches for the destination IPv4 address of the packet in its Address Resolution Protocol (ARP) cache. If the entry is not in the ARP cache, R3 sends an ARP request out of its FastEthernet 0/0 interface. PC2 sends back an ARP reply with its MAC address. R3 then updates its ARP cache with an entry for 192.168.4.10 and the MAC address that is returned in the ARP reply.
2. The IPv4 packet is encapsulated into a new Ethernet data link frame and sent out the FastEthernet 0/0 interface of R3.
3. When PC2 receives the frame, it examines the destination MAC address, which matches the MAC address of the receiving interface, its Ethernet network interface card (NIC). PC2, therefore, copies the rest of the frame into its buffer.
4. PC2 identifies the Ethernet Type field as 0x800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.
5. PC2 de-encapsulates the Ethernet frame and passes the IPv4 packet to the IPv4 process of its operating system.

The animation in the online course illustrates how R3 forwards the packet to PC2.

Path Determination (1.2.2)

This section discusses the best path to send packets, load balancing, and the concept of administrative distance.

Routing Decisions (1.2.2.1)

A primary function of a router is to determine the best path to use to send packets. To determine the best path, the router searches its routing table for a network address that matches the destination IP address of the packet.

The routing table search results in one of three path determinations:

• Directly connected network: If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the interfaces of the router, that packet is forwarded directly to the destination device. This means that the destination IP address of the packet is a host address on the same network as the interface of the router.
• Remote network: If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
• No route determined: If the destination IP address of the packet does not belong to either a connected or remote network, the router determines if there is a Gateway of Last Resort available. A Gateway of Last Resort is set when a default route is configured on a router. If there is a default route, the packet is forwarded to the Gateway of Last Resort. If the router does not have a default route, then the packet is discarded. If the packet is discarded, the router sends an ICMP Unreachable message to the source IP address of the packet.

The logic flowchart in Figure 1-29 illustrates the router packet-forwarding decision process.

Best Path (1.2.2.2)

Determining the best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or shortest path to reach that network. Whenever multiple paths to the same network exist, each path uses a different exit interface on the router to reach that network.

The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. A metric is the quantitative value used to measure the distance to a given network. The best path to a network is the path with the lowest metric.

Dynamic routing protocols typically use their own rules and metrics to build and update routing tables. The routing algorithm generates a value, or a metric, for each path through the network. Metrics can be based on either a single characteristic or several characteristics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric.

The following lists some dynamic protocols and the metrics they use:

• Routing Information Protocol (RIP): Hop count
• Open Shortest Path First (OSPF): Cisco routers use a cost based on cumulative bandwidth from source to destination
• Enhanced Interior Gateway Routing Protocol (EIGRP): Bandwidth, delay, load, reliability

The animation in the online course highlights how the path may be different depending on the metric being used.

Load Balancing (1.2.2.3)

What happens if a routing table has two or more paths with identical metrics to the same destination network?

When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. This is called equal cost load balancing. The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. The router forwards packets using the multiple exit interfaces listed in the routing table.

If configured correctly, load balancing can increase the effectiveness and performance of the network. Equal cost load balancing can be configured to use both dynamic routing protocols and static routes.

By default, Cisco routers can load balance up to four equal cost paths. The maximum number of equal cost paths depends on the routing protocol and IOS version.

EIGRP supports equal cost load balancing and is also the only routing protocol to support unequal cost load balancing. Unequal cost load balancing is when a router distributes traffic over network interfaces, even those that are different distances from the destination address.

The animation in the online course provides an example of equal cost load balancing.

Administrative Distance (1.2.2.4)

It is possible for a router to be configured with multiple routing protocols and static routes. If this occurs, the routing table may have more than one route source for the same destination network. For example, if both RIP and EIGRP are configured on a router, both routing protocols may learn of the same destination network. However, each routing protocol may decide on a different path to reach the destination based on that routing protocol’s metrics. RIP chooses a path based on hop count, whereas EIGRP chooses a path based on its composite metric. How does the router know which route to use?

Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the “trustworthiness” of the route; the lower the AD, the more trustworthy the route source. For example, a static route has an AD of 1, whereas an EIGRP-discovered route has an AD of 90. Given two separate routes to the same destination, the router chooses the route with the lowest AD. When a router has the choice of a static route and an EIGRP route, the static route takes precedence. Similarly, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1.

Table 1-5 lists various routing protocols and their associated ADs.

Table 1-5 Default Administrative Distances

Route Source	Administrative Distance
Connected	0
Static	1
EIGRP summary route	5
External BGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
External EIGRP	170
Internal BGP	200
Unknown	255

Summary (1.4)

This chapter introduced the router. The main purpose of a router is to connect multiple networks and forward packets from one network to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network.

Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The routing table is a list of networks known by the router. The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router.

Remote networks are added to the routing table in one of two ways: either by the network administrator manually configuring static routes or by implementing a dynamic routing protocol. Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable.

Dynamic routing protocols automatically adjust to changes without any intervention from the network administrator. Dynamic routing protocols require more CPU processing and also use a certain amount of link capacity for routing updates and messages. In many cases, a routing table will contain both static and dynamic routes.

Routers make their primary forwarding decision at Layer 3, the network layer. However, router interfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1. Router interfaces participate in Layer 2 processes associated with their encapsulation. For example, an Ethernet interface on a router participates in the ARP process like other hosts on that LAN.

The Cisco IP routing table is not a flat database. The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets.

Components of the IPv6 routing table are very similar to the IPv4 routing table. For instance, it is populated using directly connected interfaces, static routes, and dynamically learned routes.

Practice

The following activities provide practice with the topics introduced in this chapter. The Labs and Class Activities are available in the companion Routing Protocols Lab Manual (978-1-58713-322-0). The Packet Tracer Activities PKA files are found in the online course.

Class Activities

Class Activity 1.0.1.2: Do We Really Need a Map?

Class Activity 1.4.1.1: We Really Could Use a Map!

Labs

Lab 1.1.1.9: Mapping the Internet

Lab 1.1.4.6: Configuring Basic Router Settings with IOS CLI

Lab 1.1.4.7: Configuring Basic Router Settings with CCP

Packet Tracer Activities

Packet Tracer Activity 1.1.1.8: Using Traceroute to Discover the Network

Packet Tracer Activity 1.1.2.9: Documenting the Network

Packet Tracer Activity 1.1.3.5: Configuring IPv4 and IPv6 Interfaces

Packet Tracer Activity 1.1.4.5: Configuring and Verifying a Small Network

Packet Tracer Activity 1.3.2.5: Investigating Directly Connected Routes

Check Your Understanding Questions

Complete all the review questions listed here to test your understanding of the topics and concepts in this chapter. The appendix, “Answers to the ‘Check Your Understanding’ Questions,” lists the answers.

1. Which of the following matches a router component with its function?

   1. Flash: Permanently stores the bootstrap program
   2. ROM: Permanently stores the startup configuration file
   3. NVRAM: Permanently stores the operating system image
   4. RAM: Stores the routing tables and ARP cache

2. Which command can a technician use to determine whether router serial ports have IP addresses that are assigned to them?

   1. show interfaces ip brief
   2. show controllers all
   3. show ip config
   4. show ip interface brief

3. Which of the following commands will set and automatically encrypt the privileged EXEC mode password to “quiz”?

   1. R1(config)# enable secret quiz
   2. R1(config)# password secret quiz
   3. R1(config)# enable password secret quiz
   4. R1(config)# enable secret password quiz

4. Which routing principle is correct?

   1. If one router has certain information in its routing table, all adjacent routers have the same information.
   2. Routing information about a path from one network to another implies routing information about the reverse, or return, path.
   3. Every router makes its routing decisions alone, based on the information it has in its own routing table.
   4. Every router makes its routing decisions based on the information it has in its own routing table and the information in its neighbor routing tables.

5. What two tasks do dynamic routing protocols perform? (Choose two.)

   1. Discover hosts
   2. Update and maintain routing tables
   3. Propagate host default gateways
   4. Network discovery
   5. Assign IP addressing

6. A network engineer is configuring a new router. The interfaces have been configured with IP addresses and activated, but no routing protocols or static routes have been configured yet. What routes are present in the routing table?

   1. Default routes
   2. Remote network routes
   3. Directly connected routes
   4. No route as the routing table is empty

7. Which statements are correct regarding how a router forwards packets? (Choose two.)

   1. If the packet is destined for a remote network, the router forwards the packet out all interfaces that might be a next hop to that network.
   2. If the packet is destined for a directly connected network, the router forwards the packet out the exit interface indicated by the routing table.
   3. If the packet is destined for a remote network, the router forwards the packet based on the information in the router host table.
   4. If the packet is destined for a remote network, the router sends the packet to the next-hop IP address in the routing table.
   5. If the packet is destined for a directly connected network, the router forwards the packet based on the destination MAC address.
   6. If the packet is destined for a directly connected network, the router forwards the packet to the switch on the next-hop VLAN.

8. Which command is used to explicitly configure a local IPv6 address on a router interface?

   1. ipv6 enable
   2. ipv6 address ipv6-address/prefix-length
   3. ipv6 address ipv6-address/prefix-length eui-64
   4. ipv6 address ipv6-address/prefix-length link-local

9. Which statement is true regarding metrics used by routing protocols?

   1. A metric is the quantitative value that a routing protocol uses to measure a given route.
   2. A metric is a Cisco-proprietary means to convert distances to a standard unit.
   3. Metrics represent a composite value of the amount of packet loss occurring for all routing protocols.
   4. Metrics are used by the router to determine whether a packet has an error and should be dropped.

10. The network administrator configured the ip route 0.0.0.0 0.0.0.0 serial 0/0/0 command on the router. How will this command appear in the routing table, assuming that the Serial 0/0/0 interface is up?

    1. D 0.0.0.0/0 is directly connected, Serial0/0/0
    2. S* 0.0.0.0/0 is directly connected, Serial0/0/0
    3. S* 0.0.0.0/0 [1/0] via 192.168.2.2
    4. C 0.0.0.0/0 [1/0] via 192.168.2.2

11. How many equal-cost paths can a dynamic routing protocol use for load balancing by default?

    1. 2
    2. 3
    3. 4
    4. 6

12. What two statements correctly describe the concepts of administrative distance and metric? (Choose two.)

    1. Administrative distance refers to the trustworthiness of a particular route.
    2. A router first installs routes with higher administrative distances in its routing table.
    3. Routes with the smallest metric to the destination indicate the best path.
    4. Metrics are used by the router to determine whether a packet has an error and should be dropped.
    5. The metric is always determined based on hop count.

13. When a packet travels from router to router to its destination, what address continually changes from hop to hop?

    1. Source and destination Layer 2 address
    2. Source Layer 3 address
    3. Destination Layer 3 address
    4. Destination port
14. Describe the internal router hardware components, and outline the purpose of each.
15. Describe the router bootup process from power on to final configuration.
16. What are two important functions that a router performs?
17. Describe the steps necessary to configure basic settings on a router.
18. Describe the importance of the routing table. What purposes does it serve?
19. What are the three basic ways a router learns about networks?

20. What three pieces of information must be configured on a host to forward packets to remote networks? (Choose three.)

    1. Clock rate
    2. Default gateway
    3. DHCP server address hostname
    4. DNS server address
    5. IP address
    6. Subnet mask
21. A serial interface has been configured with an IP address and the clock rate. However, the show ip interface brief command indicates that the interface is administratively down. What must be done to correct the problem?
22. What type of IPv6 address must be configured on an IPv6-enabled interface?
23. When a computer is pinging another computer for the first time, which type of message does it send first to determine the MAC address of the other device?